The present invention relates to synchronous motors, and more particularly to the discharge resistor and excitation control used during starting of such motors.
Synchronous motors are normally started by the induction motor action of the damper winding, the field winding being unexcited during starting. The damper winding acts as a squirrel-cage rotor winding, and the motor is accelerated by the induction motor torque thus produced until it reaches a speed close enough to synchronism for the field winding to be excited with direct current at the proper instant, and the motor then pulls into step. During the starting period when the motor is running at subsynchronous speed, a relatively high voltage is induced in the field winding since it is rotating at a lower speed than the rotating field of the stator winding. In order to limit this voltage and carry the induced field current, a discharge resistor is connected across the field winding, and since this closes the circuit of the field winding, the starting torque is somewhat increased and the starting performance is improved.
In the conventional practice, this starting resistor has been separately mounted either on the rotor of the machine itself or, in some cases, externally of the motor. In either case, the starting resistors used heretofore have been relatively large and expensive and the necessary mounting provisions and space requirements have complicated the mechanical design, especially where the resistor was mounted on the rotor. This has been a particular disadvantage in the case of brushless synchronous motors, where the excitation system and starting control are carried entirely on the rotor, and the necessity for also mounting the starting resistor on the rotor has involved considerable difficulty and has resulted in rotors of large size and high cost.